It was a spur-of-the-moment thing, I'm afraid, as I usually use the structure "du genre à + infinitive" or "du genre + adjective" but not "du genre + noun" like "du genre rat de bibliothèque". Unless... the nominal phrase "rat de bibliothèque" can also serve as an attributive adjective? Or should I have phrased it otherwise?

Être du genre usage is quite flexible as it can be followed by anything that describes the expected genre, whether an adjective, a substantive, a pronoun, an interjection, a relative clause, a full sentence... although guillemets are often used when the sentence would have looked too ungrammatical otherwise.